In recent years, polymer electrolyte fuel cells have been identified as promising power sources for vehicular transportation and other applications requiring clean, quiet and efficient portable power. As a vital part of the fuel cell, proton exchange membranes (PEM)s have become a rapidly growing area of research. Until now, PEMs have been mainly limited to perfluorinated ionomer membranes such as Nafion™ developed by DuPont and similar membranes commercialized by Dow and Asahi. In spite of their outstanding properties such as excellent proton conductivity and oxidative resistance, which are essential for fuel cell application, the perfluorinated ionomer membranes are very expensive (US$800-2000/m2) and suffer from other serious drawbacks of high methanol permeation and dehydration. In order to develop alternatives to Nafion™, which would be less expensive and free from other disadvantages of perfluorinated ionomer membranes, several attempts have been recently made including synthesis of new polymer electrolytes1-5, chemical modification of available high performance polymers6-10, and blend membranes11-14. Among these studies, introducing sulfonic acid groups into the main chains of high performance polymers by sulfonation reaction is an important and widely used method for imparting polymers with proton exchange capability. Poly(aryl ether ketone)s, poly(phenylene oxide), poly(phenylene sulfide), poly(aryl ether sulfone), and polybenzimidazole are among those that have been studied. The sulfonating agents include concentrated sulfuric acid, chlorosulfonic acid, pure or complex sulfur trioxide, and acetyl sulfate. As an example, the sulfonation reaction of Victrex™ PEEK and the conductivity of sulfonated PEEK have been studied extensively7,11,15-17. The DS could be controlled by reaction time and temperature in concentrated sulfuric acid or oleum. For sulfonated PEEK with a relatively low DS of 0.65, its conductivity reaches 0.04 S cm−1 at 100° C./100% RH, higher than that of Nafion-117 measured under the same conditions.
Poly(phthalazinone ether ketone) (PPEK) is one of a new class of poly(aryl ether ketone)s under consideration for commercialization. PPEK has a very high glass transition temperature of 263° C., excellent high-temperature stability, and many other good properties. The sulfonation reactions of its copolymer, poly(phthalazinone ether sulfone ketone) (PPESK), and the nanofltration and ultrafiltration membrane properties have also been studied18-23.